Electrical insulation



w. E. GORDON 2,348,536

ELECTRICAL INSULATION May 9, 1944.

Filed June 3, 1957 x I INVENTOR. Lace 11.. Enmr'flun flf . ATTORNEY.

rsrenieumsyanm 2,348,536 ELECTRICAL INSULATION Wallace E. Gordon, Wilmington, Del., assignor to E. I. du Pont de Nemonrs & Company, Wilmington, Del., a corporation of Delaware Application June 3, 1937, Serial No. 146,321

22 Claims.

The invention herein described relates to electrical insulation and particularly to the use of synthetic linear condensation polyamides as electrical insulating materials.

An object of this invention is to provide an improved type of electrical insulating material. A further object is to prepare improved insulated electrical conductors. A still further object is to insulate electrical conductors by mean of a coating or wrapping of great strength, good pliability, good thermal stability, high melting point, good resistance to water and oil. and good aging qualities. Other objects will become apparent as the description proceeds.

These objects are accomplished by fashioning an electrical insulator from asynthetic linear condensation polyamide. More specifically these objects are accomplished by coating or otherwise covering an electrical conductor with a composition comprising a synthetic linear condensation polyamide. In its preferred embodiment, therefore, this invention comprises insulated electrical conductors in which the insulating media comprises a synthetic linear condensation polyamide.

Numerous types of materials have been described as insulating materials for electrical conductors. Among the more commonly employed materials for this purpose are: rubber, various synthetic resins, and cellulosic materials. While each of these materials has certain specific advantages, they suffer from certain defects. For example, while rubber has good insulating properties, is quite flexible, and has good water resistance, it is softened by oils and deteriorates on exposure to air, particularly if ozone is present, becoming hard and inelastic. Phenolformaldehyde resins are quite strong, are resistant to water, and have good aging qualities, but they are not sufllciently flexible for certain purposes and require special baking treatment for satisfactory application. On the other hand, most other resinous materials are characterized by low softening temperatures and poor resistance to solvents. Thus, while meta styrene has excellent electrical properties and good resistance to water it has a low softening temperature and is softened by oils. The cellulosic materials are light and flexible but are in general characterized by poor water resistance and high inflammability.

I have now found that synthetic linear condensation polyamides are relatively free from the defects mentioned above in connection with previously described insulating materials. As

far a I am aware no product possesses to as high a degree the combined properties of toughness, pliability, thermal stability, high melting point, resistance to water, resistance to oils, good aging qualities (even under severe weather conditions), and at the sam time the satisfactory electrical insulating properties of the synthetic linear condensation polyamides herein described. Furthermore, I have found that these polyamides can be readily applied to electrical conductorseither as a continuous coating or as a wrapping material in the form of a sheet, filament, or fabric.

Before describing in detail the preparation of insulated electrical conductors according to this invention, further reference should be made to the synthetic linear condensation polyamides employed for this purpose. The term synthetic linear condensation polyamide" implies that the polyamides are long chain, 1. e., two-dimensional polymers prepared by artificial means. In other words, they are derived from bifunctional monomeric reactants. The polyamides are of two typ s, Ithose derived from polymerizable monoaminomonocarboxylic acids and their amide-forming derivatives, and those derived from suitable diamines and suitable dicarboxylic acids or amide-forming derivatives of dibasic carboxylic acids. On hydrolysis with strong mineral acids the polyamides yield the amideforming derivatives from which they are derived, the constituent containing the amino group or groups being obtained in the form of a mineral acid salt. The preparation of these high molecular weight synthetic linear condensation polyamides known as superpolyamides is described in detail in United States Patents 2,071,250 and 2,071,253.

Of the synthetic linear condensation polyamides the most useful as insulating materials for electrical conductors are those having an intrinsic viscosity above 0.4, where intrinsic viscosity is defined as where i is the viscosity of a dilute meta-cresol solution of the polyamide divided by the viscosity of meta-cresol in the same units and at the sam temperature and C is the concentration in grams of polyamide per cc. of solution. In general the strength and toughness of the polyamide increase with increase in intrinsic viscosity, polyamides having intrinsic viscosities between 0.6 and 2.0 being particularly useful in the present invention.

As disclosed inlthe above mentioned patents, highly condensed synthetic linear condensation poiyamides are capable of being formed into useful filaments, films, and the like. In general fiber-forming properties are not pronounced unless the polyamide has an intrinsic viscosity of at least 0.4. Filaments derived from these polyamides are characterized by the fact that they are converted into oriented fibers of greater utility than the original filaments upon application of stress herein referred to as "cold drawing. Molecular orientation may also be obtained by other methods of cold working as for instance by the application of compressive stress through cold rolling. It. is within the scope of this present invention to use the polyamide filaments as electrical insulating materials in either the oriented or unoriented form. It is also within the scope of this invention to coat a wire with a a polyamide and then subject both the wire and .the coating to cold drawing. Polyamide films can also be used in the oriented form: in fact,

the oriented products are characterized by superior strength and elasticity than the unoriented products.

For the most part, synthetic linear condensation polyamides derived from straight, chain reactants, e. g., polymethylene diamines and polymethylene dicarboxylic acids are characterized by high melting points, generally above 150' C.

and often above 225' 0., are substantially thermally stable even in the neighborhood of their -melting point, are microcrystalline in character,

are not hydrolyzed by water, and are substantially insensitive to most organic solvents, including oils. This extraordinary resistance of the polyamides to solvents and chemical agents is further pointed out in Patent 2,130,523. An additional instance of the chemical lnertness of the polyamides is their resistance to such highly active agents as sulfur dioxide and dichlorodifiuoromethane. Furthermore, they have low densities (1.0 to 1.2), are tough, and unusually strong and elastic. Thus, the strength of fibers derived from these polyamides is superior to that of cotton and fully as good as that of silk. In film or sheet form the strengthof the polyamides generally ranges from 5000 to 20,000 pounds per square inch, depending upon their method of preparation.

These desirable qualities together with the good electrical properties shown by the tests given in the accompanying table indicate the exceptionally high value of the polyamides described herein as insulating materials. In this table the electrical properties of two typical synthetic polyamides are compared with a phenol-formaldehyde resin, one of the more common synthetic insulating materials. The data for the polyamides were obtained from measurements made on films of the products. The data given for the phenol-formaldehyde resin are taken from the October 1936 number of "Modern Plastics."

In the table polyamide A represents polyhexamethylene adipamide, derived from hexamethylenediamine and adipic acid, and polyamide 3 represents polydecamethylene diphenylolpropane diacetamide, a polyamide derived from decamethylenediamine and p.p'-diphenyiolprupane diacetic. acid (HCQCCHaOCsH4C(CHsl lCtHAOCHzCOOH) As already indicated, synthetic linear condensation poiyamides can be employed as insulating materials, for electrical conductors in a variety or ways. One method consists in passing the electrical conductor through the molten polyamide. When this is done, it is advantageous to cool the coated conductor rapidly. e. g., by immersion in water. Another method consists in coating or spraying the conductor with a solutionof the polyamide in a suitable solvent and removing the solvent by evaporation or by coagulating methods. when the polyamide is applied in a solvent it is usually desirable to apply successive thin coatings rather than one heavy coating, since this results in a more uniiorm product. Still another method consists in wrapping the electrical conductor with filaments.

yarns, fabrics, ribbons, films, and the like prepared from the polyamides. When this method is used it is desirable to use the oriented products. Suitable materials, e. g., cloth and paper. impregnated with the polyamides can also be employed as insulating materials. Other methods known to the art may also be employed. It is within the scope of this invention to surround the polyamide coating or covering with another material, e. g., wax, pitch, or lead, but this is generally unnecessary in view of the remarkable properties of the polyamides. The polyamides may be used as a, covering for other insulating materials.

In the accompanying drawing Fig. 1 represents an electrically insulated conductor with the previously described polyamide insulation;

Fig. 2 is a fragmentary view of a stator of a motor embodying the invention; and

Fig. 3 is a section between the teeth of the stator showing one of the lined slots therein.

In Fig. 1 the numeral I indicates an electrically conducting wire or cable provided with an adherent coating of synthetic linear polyamide insulation. In the application of the invention to a motor as shown in Figs. 2 and 3, the numeral 4 indicates the stationary portion of a motor having a frame 5 in the form of a ring which supports the yoke shaped magnetic core 5' which is provided with slots having liners 1 within which the insulated conductors of the motor are wound. The numeral 8 indicates the turns of the insulated motor-running conductor windings and the numeral 9 a layer of insulated starting conductor windings, A separator I2 is disposed between these windings as shown in Fig. 3. The coil ends can be joined in accordance with previously known construction by twisting within the sleeves I I. The liners I, separators l2, and insulation on the conductors are all preferably composed of synthetic linear polyamide.

The following specific examples illustrate the invention in greater detail:

Example I A solution of poiyhexamethylene adipamide of intrinsic viscosity 1.69 was prepared by dissolving 20 parts by weight of the polyamide in 75 parts by weight of formic acid. Through this solution was then passed a blasting cap leg wire (#22 copper wire). On leaving the solution the coated wire passed through a round orifice having a diameter such that the excess solution was removed from the wire leaving a coating of the desired thickness. After the excess solution was removed, the coated wire was dried at 100 C. By varying the concentration of the bath and/or the diameter of the orifice it is possible to obtain coatings of 0.001 to 0.006 inch in thickness after drying.

Colored coatings were applied in a similar manner by adding suitable dyes to the poiyamide solution.

Example II Copper wire was coated with polyhexamethylene adipamide of intrinsic viscosity 0.87 by passing the wire through the molten polyamide under oxygen-free conditions at approximately 280 C. The wire entered the molten bath through a suitable guide and left the bath through an orifice in a direction perpendicular to the surface of the bath. The coated wire while still quite hot was x then passed through a water bath. 'In this manner coatings of about 0.01 inch in thickness were obtained. The coating so applied was unusually hard and tough and adhered very well to the copper wire.

Example III Example IV A small copper wire was coated with an interpolyamide of intrinsic viscosity 1.01 prepared by heating under condensation polymerization conditions equimolecular amounts of hexamethylene diammonium adipate (salt derived from hexamethylenediamine and adipic acid) and decamethylene diammonium sebacate. The coating was applied from a solution of the interpolymer in 98% formic acid after the manner described in Example I. The coating showed excellent adhesion and pliability. The wire could be bent repeatedly without loosening or cracking the coating.

It is to be understood that the foregoing examples are illustrative of the manner of carrying out the invention and are not to be considered as limitative. Other types of synthetic linear condensation polyamides can be employed in addition to those cited. A valuable class of polyamides for electrical insulation purposes are those derived from one or more diamines of formula NHzCHzRCHzNHz and one or more dicarboxylic acids of formula HOOCCHzR'CHzCOOH or amide-forming derivatives thereof, in which R and R. are divalent hydrocarbon radicals free from olefinic and acetylenic unsaturation and in which R has a chain length of at least two carbon atoms. An especially valuable group of polyamides within this class are those in which R is (CH2): and R is (CI-I2) where a: is at least two. Furthermore, polyamides derived from polymerizable monoaminomonocarboxylic acids or amideforming derivatives thereof are useful as electrical insulating materials. Interpolyamides, i. e., polymers derived from a mixture of polyamideforming reactants capable of yielding more than one polymer if reacted in suitable combinations,

in the manufacture of insulated wire.

are especially useful since they are characterized by high pliability and elasticity. Mixtures of preformed polyamides are also useful. Typical examples of polyamides useful for the preparation of insulated electrical conductors and non-conducting electrical parts are listed below:

Polyamide derived from- M. P., C. Ethylenediamine and sebacic acid 254 Tetramethylenediamine and adipic acid- 278 Hexamethylenediamine and p'inic acid-..

Hexamethylenediamine and diphenic acid 157 Hexamethylenediamine and 1,2-cyclohexanediacetic acid 255 Octamethylenediamine and adipic acid 235 Octamethylenediamine and sebacic acid- 19'! Decamethylenediamine and carbonic acid 200 Decamethylenediamine and oxalic acid-.. 229 Decamethylenediamine and adipic acid..- 230 Decamethylenediamine and p-phenylene diacetic acid 242 p-Xylylenediamine and sebacic acid- 268 Decamethylenediamine and p,p'-

diphenylolpropane diacetic acid 105 Methylhexamethylenediamine and adipic acid 3,3'-diamino propyl ether and adipic acid 6-aminocaproic acid 203 9-aminononanoic acid The melting points indicated in the table were determined by placing fine particles of the polyamide on a heated metal block in the presence of air and noting the fusion temperature.

The electrical insulating compositions of this invention need not necessarily consist solely of the polyamides. It is within the scope of this invention to use softening or plasticizing agents in conjunction with the polyamides. Fillers may also be added. The fillers may be organic, such as wood flour, paper, fibrous materials, etc., or inorganic, such as mica, asbestos, glass, etc. Although the polyamides do not burn readily, they may be compounded if desired with flame retardants, such as halogen-substituted hydrocarbons, phosphates, etc.

As indicated by the examples, an important application of the compositions of this invention is A large part of such wire as now commercially manufactured is rubber covered, thev rubber being extruded and cured on the wire. For this use the polyamides are superior to the materials previously employed particularly with respect to oil and ozone resistance, properties which result in a longer life. The polyamidescan be extruded by conventional rubber tubing machines and require no curing.

The invention is applicable to the manufacture of insulated conductors of all kinds. As examples might be mentioned high tension cables, small household wire in which the Underwriters specifications will not permit the use of rubber, and flexible cables for telephones, automobile ignition systems, neon signs, and other high voltage uses.

Example III illustrates the use of a polyamide as insulating material in the form of a self-supporting sheet. Sheet insulation of this kind serves as an improved substitute for such materials as cloth, paper, regenerated cellulose, etc. 'Polyamide sheets or ribbons may be used to advantage in insulating wires for magnets, for coils in motors and generators, e. g., dynamo electric machines, for radio and telephone coils, and in exterior and interior wires of many kinds. Larger sheets may be used for the manufacture of condensers in which the molding properties of the polyamide are advantageous in that the condenser may be compressed, with the aid of heat to a rigid mass, thereby eliminating air spaces. The insulating composition in sheet form may also be used in so-called slot insulation of motors and generators wherein the polyamide sheet forms a liner for the slots which receive the motor or generator windings, and as coil separators in transformer windings. These motors, due to the previously mentioned chemical inertness oi the insulation toward chemical agents, are well adapted for use in refrigerators since the polyamide is resistant to the action of sulfur dioxide, dichlorodifiuoromethane, and other refrigerants.

The fact that the polyamides can be readily molded and remain fusible makes them particularly suited to the manufacture of solid insulation and non-conducting electrical parts. Thus, the compositions may be used in the molding of cross-sections such as are impossible of production wtih the thermal-setting molding compositions, e. g., of the phenol-aldehyde or urea-aldehyde type. For this purpose the compositions may be manufactured in the form of cylinders, tubes, sheets, etc. for machining, or may be molded directly into any of the numerous forms required for solid insulation, e. g., electric light fixtures, washers, receptacles, and plugs. The polyamide compositions may also be used in the manufacture of larger articles in which a nonconducting material is required as, for example, telephone receivers and transmitters, switchboards, radio panels, etc. The compositions have the important advantage that they may be produced in a transparent or light colored form as well as the dark colored form to which most materials previously employed are restricted.

The synthetic linear condensation polyamldes may also be used as a binder or impregnating agent in the manuf ture of insulating materials. For this use the p yamides show an advantage over materials now employed for this purpose as regards higher melting point, greater hardness, increased mechanical strength, and increased durability. For this purpose the polyamide compositions may be employed in the impregnation of paper, cotton fabrics, silk, and cotton windings for cables, wires, condensers, transformer cords, motors, and generator coils. The compositions may be used both to increase the insulating properties of the impregnated material and to form, as in the case of coils, a mechanically rigid body as is now the practice in the manufacture of electrical equipment.

As already indicated, the polyamides may be used in conjunction with mica. This combination is particularly useful in forming high tension ini said polyamide is the reaction polyamide obtained sulation. The compositions may likewise be employed to bind the metals in ma neti c e where a non-conducting binder is required in order to cut down eddy current.

The insulating materials described herein are characterized by good toughness, flexibility, strength, durability, abrasion resistance, thermal stability, resistance to water and oils, adhesion to metals, and good aging qualities. These properties together with their low power factor, high volume resistivity, high surface resistivity, and high breakdown strength make these compositions useful as electrical insulating materials in a wide variety of applications, These compositions, as has been pointed out above, possess marked advantages over previously described electrical insulating compositions. Owing to these advantages, and particularly in strength, less polyamide coating is necessary in insulating conductors than in the case of most other insulators. This means that if the polyamide is applied in the form of ribbons, fibers, or the like fewer plies are necessary. In addition to being useful in insulating electrical conductors the polyamide compositions herein described are useful in the manufacture of solid insulation and non-conducting electrical parts.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

I claim:

1. An electrical conductor provided with an adherent continuous coating of synthetic linear crystalline polyamide.

2. Wire having applied thereto an insulating covering comprising a synthetic linear condensation polyamide in the form of a thin, adherent, continuous fllm.

3. The product set forth in claim 19 in which product of a diamine and a substance of the group consisting of dicarboxylic acids and amide-forming derivatives of dicarboxylic acids.

4. The product set forth in claim 19 in which said polyamide is one obtainable from a polymerizable amino acid.

5. A process for preparing an insulated electrical conductor, comprising coating 9, wire with a synthetic linear polyamide and then subjecting both the wire and the coating to cold drawing.

6. A dynamo-electric machine having a member comprising a magnetic core and windings of an electrical conductor about said core, said conductor being insulated with a composition comprising a superpolyamide.

7. A motor having a member comprising a magnetic core, coil windings about said core, and insulation on said windings comprising a superby reacting hexamethylene diamine with adipic acid.

8. A motor having a stationary member comprising a core provided with a plurality of slots, a liner for each of said slots formed of a composition comprising a superpolyamide, and windings of an electrical conductor within the lined slots, said conductor being insulated with a composition comprising a superpolyamide.

9. A motor insulated in its windings with insulation comprising a superpolyamide.

10. A motor coil the windings of which are insulated with a superpolyamide.

11. A dynamo-electric machine of the type provided with slot insulation in which the insulation contained in the slots comprises a superpolyamide.

12. An insulated electrical conductor the insulation of which comprises a superpolyamide and a fibrous filler.

13. A dynamo-electric machine the windings of which are insulated with insulation comprising a superpolyamide and a fibrous filler.

14. An electric motor provided with slots for receiving the motor windings, a liner for the slots formed from a composition comprising a superpolyamide, and windings of an electrical conductor within the slots, said conductor being insulated with a composition comprising a superpolyamide.

15. The insulated electrical conductor set forth in claim 12 in which said fibrous filler is asbestos.

16. A process for preparing an insulated electrical conductor, comprising coating a wire with a synthetic linear polyamide and then subjecting the coating to cold drawing.

17. An electrically insulated conductor which comprises insulating material covered by another insulating material comprising a superpolyamide.

18. An electrically insulated conductor in which the insulating material comprises superpolyamide and glass.

19. An electrical conductor comprising a conducting metal core surrounding by a covering comprising a synthetic linear polyamide.

20. An electrical conductor comprising a metal wire provided with a covering comprising a synthetic linear polyamide possessing crystalline orientation as shown by X-ray diffraction patterns.

21. An electrical conductor comprising a metal wire provided with a covering comprising a synthetic linear condensation superpolymer possessing crystalline orientation as shown by X-ray diffraction patterns.

22. The electrical conductor set forth in claim 19 in which said coating comprises polyhexamethylene adipamide.

WALLACE E. GORDON. 

